1. Field of the Invention
This invention relates to the field of optical systems and other precision devices, and more particularly to a method and construction for accurately and securely retaining a lens or other optical element in a cell or other holding structure. The invention is particularly suitable for retention of large, heavy optical elements that are prone to become displaced in the course of shipment or usage, thereby upsetting an initial condition of accurate optical alignment of the optical elements.
2. Description of Related Art
In the art of optical design, accurate alignment of the optical elements (e.g. the objective lens) in an optical system is critical to achieving the desired performance of the system. This is particularly true in the case of high performance optical systems, such as high resolution photographic optical systems employed in the field of aerial reconnaissance. When such systems are designed and manufactured, great care is taken to insure that the lens and other optical elements are installed into the camera housing such that the lens is correctly aligned relative to the lens cell or other structure to which the lens is mounted. Ideally, such aligned condition is maintained during shipment from the manufacturer to the customer, and thereafter during use.
When the lens in question is relatively small or light weight, known lens retaining methods have generally been adequate to keep the lens retained in the surrounding mechanical structures during shipping and subsequent use. However, when the optical elements are of an increased size, or of an increased weight (due to either large size, doping of the glass with heavy elements, or both) and only flat contact surfaces exist, known lens retaining methods have a potential for failure. Mechanical shocks and vibration during shipment and use are more likely to upset the initial condition of accurate alignment of such heavy or large optical elements. Any deviation of the lens alignment from the initial, aligned condition can seriously degrade the performance of the system. For example, if the system is a high-resolution aerial reconnaissance camera system to be flown in a military aircraft, a non-aligned condition of the objective lens can result in substantial loss of resolution of the resulting photographs or electro-optical imagery.
FIG. 1 is a cross-sectional view of a prior art set up for retaining an optical element 10 having flat contact surfaces 16A and 16B in a cell 12; a plano-convex lens element is shown as an example. Such a component may be called a flat-flat element. The cell 12 provides a holding structure for the element 10 in a nominally aligned condition relative to an optical and mechanical axis 14. In the typical situation, the lens element 10 has a flat/flat contact interface at its front and rear contact areas of the element, as indicated as 16A and 16B. Normally, when the optical element 10 is centered in its seat 18 in the lens cell 12, the small space 20 around the periphery of the element 10 is filled with an elastomeric material such as RTV. After curing of the elastomer, the element 10 is retained by a threaded retaining ring 22 which installs onto threads 24 provided in the cell 12, and the retaining ring 22 is secured to the cell 12 by staking or other means. When a sufficient transverse force is applied to the element 10, by say, mechanical shock, the element 10 may overcome the frictional forces between the element 10 and the retaining ring 22 and between the element 10 and the seat 18. Any significant transverse displacement away from the mechanical axis 14, even though small in terms of magnitude, can upset the state of optical symmetry and alignment and degrade the optical performance of the complete lens system. This is particularly so in the case of high-performance lens systems such as are found in aerial reconnaissance camera systems and in optical sights for weapon systems.
Another prior art retaining system not used with flat-flat elements, this one having a floating ring, is illustrated in FIG. 2. Like components illustrated in FIGS. 1 and 2 are given the same reference numerals. Referring to FIG. 2, the optical element 10, shown by way of example, has a convex surface 11 and is centered within its seat 18 in the lens cell 12. A floating ring 26 is installed over the outer convex optical surface 28 of the element 10 and a threaded retaining ring 22 is positioned on the floating ring 26. In this situation, after the element 10 has been centered on its seat 18 with respect to the optical axis 14, and the elastomer has been poured and cured around the element in the space between the element 10 and the seat 18, the two-ring system 26 and 22 is installed on the element 10. As the threaded retaining ring 22 is tightened on the floating ring 26, the floating ring adjusts its position by rolling around the spherical optical surface 28 of the element 10 until the outer surface 30 of the floating ring 26 becomes parallel to the inner or rear surface 32 of the threaded retaining ring 22. This action will not upset the alignment of the element, and will provide positive restraining forces on the element 10.
The method of FIG. 2, however, cannot be used for elements with a flat-flat configuration (such as shown in FIG. 1). The reason is that the floating ring 26 would not be able to find a position in which it intimately contacts both the flat surface 16A (FIG. 1) of the element 10 and the flat surface 32 of the threaded retaining ring 22 due to the effective radius of the flat surface of surface 16A being infinite.
The present invention addresses the problem of how to retain a flat-flat optical element, particularly one that is relatively large and/or heavy, within its associated cell such that the condition of accurate optical alignment is maintained during shipment and subsequent use. As such, the present invention presents a substantial improvement over the constructions shown in FIGS. 1 and 2.